Silver halide photography usually involves the exposure of silver halide with light in order to form a latent image that is developed during photographic processing to form a visible image. Silver halide is intrinsically sensitive only to light in the blue region of the spectrum. Thus, when silver halide is to be exposed to other wavelengths of radiation, such as green or red light in a multicolor element or infrared radiation in an infrared-sensitive element, or when it is desired to augment the intrinsic sensitivity to blue light, a spectral sensitizing dye is required. Sensitizing dyes are chromophoric compounds (usually cyanine dye or merocyanine compounds) that are adsorbed to the silver halide. They absorb light or radiation of a particular wavelength and transfer the energy to the silver halide to form the latent image, thus spectrally sensitizing the silver halide.
Merocyanines are well-known as spectral sensitizing dyes. For example, U.S. Pat. No. 2,177,402 describes merocyanines having a thiazolone nucleus. Some of these dyes can be represented by the formula: ##STR2## where R is alkyl or aryl and Z is a heterocycle.
U.S. Defensive Publication T888,016 describes merocyanine sensitizing dyes, some of which can be represented by the formula: ##STR3## where R is alkyl or aryl and Z is a heterocycle.
Many spectral sensitizing dyes, however, suffer from a number of problems. Many dyes cause undesirable post-processing stain in photographic elements. Other dyes, such as the above-described dyes of U.S. Pat. No. 2,177,402 and U.S. T888,016, may be limited in the level of sensitivity (i.e., photographic speed) imparted to silver halide emulsions.
It is thus an object of the present invention to provide a merocyanine spectral sensitizing dye that provides high sensitivity to silver halide emulsions without causing objectionable post-processing stain.